1. Field of the Invention
The present invention relates to a switching power supply device, and more particularly, to a switching power supply device employing an RCC (ringing choke converter) system.
2. Description of the Related Art
In general, for equipment and apparatus such as VTR, facsimile equipment, and so forth, a stable direct current voltage is required. In order to supply a stable direct current voltage from a commercial alternating current power supply, is widely used a switching power supply device employing an RCC system of which the configuration is relatively simple and the efficiency is high.
In FIG. 4, there is shown a conventional RCC system switching power supply device. In FIG. 4, the switching power supply device 1 is formed of an input circuit 2, a DC--DC converter circuit 3, a voltage detecting circuit 4, and a control circuit 5.
The input circuit 2 is made up of a rectifying diode bridge DB coupled to an AC power supply. A fuse F is provided between the AC power supply and the diode bridge DB. A line filter LF and a smoothing capacitor C1 are connected across the output terminals of the diode bridge DB.
The DC--DC converter circuit 3 is made up of a transformer T having a primary winding N1, a secondary winding N2 opposite in polarity to the primary winding N1, and a feedback winding Nb having the same polarity as the primary winding N1, FET Q1 as a switching element, connected in series with the other end of the primary winding N1, a starting-up resistor R1 connected between one end of the primary winding N1 and the gate of FET Q1 as a controlling terminal, a rectifying diode D1 connected in series with the other end of the secondary winding N2, and a smoothing capacitor C2 connected between the cathode of the diode D1 and one end of the secondary winding N2.
A voltage detecting circuit 4 provided on the output side of the DC--DC converter circuit 3 is made up of a resistor R2, a light emitting diode PD1 on the light emitting side of a photocoupler PC1, a shunt regulator Sr and resistors R3, R4. The resistor R2, the shunt regulator Sr, and the resistors R3, R4 are connected in series with one another, and provided in parallel to the capacitor C2 of the DC--DC converter circuit 3. The resistors R3, R4 are connected in series with each other, and provided in parallel to the capacitor C2. A connection of the resistors R3, R4 is connected to the shunt regulator Sr.
The control circuit 5 is made up of a resistor R5 and a capacitor C3 connected in series with each other, provided between one end of the feedback winding Nb and the gate of FET Q1, a transistor Q2 as a controlling element, connected between the gate of FET Q1 and the other end of the feedback winding Nb, a diode D2 with its anode connected to the one end of the feedback winding Nb, a resistor R6 connected between the cathode of the diode D2 and the base of the transistor Q2 as the controlling terminal, a capacitor C4 connected between the base of the transistor Q2 and the other end of the feedback winding Nb, a resistor R7 connected in parallel to the capacitor C4, a resistor R8 and a phototransistor PT1 on the light reception side of the photocoupler PC1 connected in series with each other, provided between the cathode of diode D2 and the base of the transistor Q2, a diode D3 with its cathode connected to one end of the feedback winding Nb, a resistor R9 and a capacitor C5 connected in series with each other, provided between the anode of the diode D3 and the other end of the feedback winding Nb, and a resistor R10 connected between a connection of the resistor R9 with the capacitor C5 and the base of the transistor Q2.
The operation of the switching power supply device 1 shown in FIG. 4 will be now described with reference to the graph of FIG. 5 showing the change of voltage and current in the relevant respective portions of the switching power supply device 1. In FIG. 5, Vgs, V1, I1, Vds, Vbe2, Vb, V2, and I2 represent the gate-source voltage of FET Q1, a voltage applied to the primary winding N1, a current flowing in the primary winding N1, the drain-source voltage of FET Q1, the base-emitter voltage of the transistor Q2, a voltage produced in the feedback winding Nb, a voltage produced in the secondary winding N2, and a current flowing in the secondary winding N2, respectively. ON, OFF written in the upper portion of the graph represent the timing when FET Q1 is turned from OFF to ON (hereinafter, referred to as "turn-on") and from ON to OFF (hereinafter, referred to as "turn-off"), respectively.
First, the instant that the power supply is turned on for starting up, FET Q1 is off, so that no current flows in the primary winding N1. However, a current flows into the internal capacitor formed between the gate-source of FET Q1, through the starting-up resistor R1. Thereby, the gate-source voltage of FET Q1 is raised. At the time when the voltage Vgs exceeds the threshold of FET Q1, FET Q1 begins to be turned on, and then, the drain-source voltage Vds of FET Q1 becomes nearly zero. As a result, a voltage from the power supply is applied to the primary winding N1 of the transformer T, causing the current Ti to begin to flow. Thereby, voltages Vb, V2 are produced in the feedback winding Nb and the secondary winding N2, respectively. The voltage Vb produced in the feedback winding Nb makes a current flow into the gate of FET Q1 from the feedback winding Nb through the resistor R5 and the capacitor C3. This accelerates the rising-up of the gate-source voltage Vgs of FET Q1, so that FET Q1 is completely turned on. In this case, no current flows in the secondary winding N2, since voltage V2 produced in the secondary winding N2 is in the backward direction with respect to the rectifying diode D1.
When FET Q1 is turned on and a voltage Vb positive in polarity is produced, the capacitor C4 is charged through the diode D2, the resistor R6, and the resistor R8 and the phototransistor PT1 as described below, so that the voltage across the opposite ends of the capacitor C4, namely, the base-emitter voltage Vbe 2 of the transistor Q2 is raised. In this case, the charging speed (time constant) is determined by the values of the resistors R6, R7, and R8, and the capacitor C4, and the phototransistor PT. When the base-emitter voltage Vbe2 of the transistor Q2 is raised to exceed a threshold Vth of the transistor Q2, the transistor Q2 is turned on. When the transistor Q2 is turned on, the collector-emitter voltage of the transistor Q2, namely, the gate-source voltage Vgs of FET Q1 becomes nearly zero, acting to turn off FET Q1.
When FET Q1 begins to turn off, the voltage V1 applied to the primary winding N1 becomes zero, and also the current I1 flowing in the primary winding N1 becomes zero. However, voltages in the primary winding N1, the secondary winding N2, and the feedback winding Nb, reverse in polarity to those applied until then, are produced, due to magnetic energy stored in the transformer T, caused by the current I1 which has flown in the primary winding N1 in the on-state of FET Q1. A voltage is produced in the primary winding N1, which is n (ratio of turns of the primary winding to the secondary winding) times higher than the voltage V2 produced in the secondary winding N2, having the reverse polarity. The current I2, caused by the voltage V2 produced in the secondary winding N2, having a reverse polarity, flows through the diode D1, and is smoothed in the capacitor C2 to be outputted. The voltage Vb generated in the feedback winding Nb, having the reverse polarity, rapidly absorbs the electric charge from the internal capacitor formed between the gate and the source of FET Q1, through the capacitor C3 and the resistor R5, completely turning off FET Q1. At the same time, the feedback winding Nb absorbs the electric charge stored in the capacitor C4, through the resistors R10, R9 and the diode D3. However, since a voltage reverse in polarity is applied to the capacitor C4, the capacitor C4, after it is discharged, is charged in the reverse direction, and the base-emitter voltage Vbe of the transistor Q2 is negatively biased, resulting in the turn-off of the transistor Q2. Thus, the transistor Q2 turns on only at the instant that it triggers the turn off of FET Q1.
While FET Q1 is off, the current I2 flowing in the secondary winding N2 is reduced stepwise with release of the magnetic energy from the transformer T, and finally becomes zero. When the current I2 flowing in the secondary winding N2 becomes zero, the voltages V2 and Vb generated in the secondary winding N2 and the feedback winding Nb, respectively, tend to be damped, oscillating on the baseline of zero voltage. In this case, the voltage, of which the reverse polarity is temporarily changed to the positive polarity in the feedback winding Nb, is called a kick voltage. When the kick voltage is generated in the feedback winding Nb, a current flows into the internal capacitor formed between the gate and the source of FET Q1, from the feedback winding Nb through the resistor R5 and the capacitor C3, increasing the gate-source voltage Vgs of FET Q1. If the kick voltage is higher than a predetermined value, the gate-source voltage Vgs exceeds a threshold to turn FET Q1 on. At this time, less current flows in the starting-up resistor R1, since the starting resistor R1 is set to a high resistance. Accordingly, the current flowing in the starting-up resistor R1 has no function of turning FET Q1 on. When FET Q1 is turned on, the voltages V2 and Vb generated in the secondary winding N2 and the feedback winding Nb, respectively are forced toward the positive polarity, so that the oscillation of the voltage is forcedly stopped.
After the forced stopping, the same operation as in the starting-up is repeated. That is, FET Q1 is turned on and off repeatedly, and thus, the switching power supply device operates.
Lastly, the voltage stabilization operation will be described. The output power is divided by the resistors R3, R4 to be detected, and is inputted into the shunt regulator Sr. The shunt regulator Sr compares the inputted voltage with its internal reference voltage, and makes the current flow which is in correspondence to the difference between the compared voltages.
In case a load (not shown) connected to the switching power supply device 1 is light (draws low current) and the output voltage is raised, the voltage at the connection between the resistors R3, R4 is increased. As a result, the input voltage to the shunt regulator Sr is increased, making a larger current start to flow. With an increased current flowing in the shunt regulator Sr, the current flowing in the light emitting diode PD1 of the photocoupler PC1, which is connected in series with the shunt regulator Sr, is increased, thereby increasing the quantity of light emitted from the light emitting diode PD1. With increase of the quantity of light emitted from the light emitting diode PD1, a current flowing in the phototransistor PT1 of the photocoupler PC1 is increased. The current flowing in the phototransistor PT1, together with the current flowing in the resistor R6 when the voltage Vb generated in the feedback winding Nb is positive in polarity as described above, acts to charge the capacitor C4. Accordingly, when the current flowing in the phototransistor PT1 is increased, the time taken to charge the capacitor C4 is shortened. As a result, the time taken until the transistor Q2 is turned on is shortened, and also the time until FET Q1 is turned off, that is, the time while the FET Q1 is on, is shortened. The short on-state time-period of the FET Q1 reduces the magnetic energy stored in the transformer T and the voltage V2 in the secondary winding N2, resulting in lowering of the output voltage. The time while FET Q1 is off is shortened in proportion to the on-state time-period of FET Q1. Accordingly, the switching frequency of the switching power supply device 1 is increased in correspondence to a decrement in the time-period while FET Q1 is on and off.
To the contrary, when the load (not shown) connected to the switching power supply device 1 is heavy and draws higher current and the output voltage is reduced, the current flowing in the phototransistor PT of the photocoupler PC is decreased, so that the charging time of the capacitor C2 is prolonged. The time until FET Q1 is turned off, that is, the time while FET Q1 is on, becomes longer, the voltage V2 produced in the secondary winding N2 is increased, and the output voltage is increased. Since the on-state time-period of FET Q1 becomes longer, the switching frequency of the switching power supply device 1 is reduced.
In the above-described manner, the switching power supply device 1 attempts to stabilize the voltage.
At the time when FET Q1 is turned on and off, there exists, for a short time, a state wherein a voltage is applied across the drain-source of FET Q1 with a current flowing. At this time, a loss is caused in FET Q1 (hereinafter, referred to as switching loss). The switching loss, generated every time FET Q1 is turned on and off, is increased in proportion to the switching frequency. Accordingly, especially when the load is light, the switching frequency is increased, causing a problem that the efficiency of the switching power supply device 1 deteriorates.